FIGS. 1a-1e illustrate a conventional method for making a printed circuit board. As illustrated in FIG. 1a, a lower film 120 having a circuit pattern is placed on a lower support plate 110, such as a glass platen, having registration pins 115. The lower film has holes corresponding to the registration pins 115, and the registration pins 115 are inserted into the holes of the lower film 120. As illustrated in FIG. 1b, an upper film 130 is then placed on top of the lower film 120. The upper film 130 also has holes corresponding to the registration pins 115, and the registration pins are inserted into the holes of the upper film 130. Next an upper support plate 140, such as a glass platen, is placed on top of the upper film 130. Vacuum is then drawn between the lower support plate 110 and the lower film 120 and between the upper support plate 140 and the upper film 130. As a result, the lower film 120 is pressed against the lower support plate 110 and the upper film 130 is pressed against the upper support plate 140. The upper film 130 and the upper support plate 140 are then pulled away from the lower film 120 and the lower support plate 110 (FIG. 1c). A copper-clad circuit panel 100 coated with photosensitive material is then placed on top of the lower film 120 (FIG. 1d), and the upper film 130 and the upper support plate 140 are pushed down against the panel (FIG. 1e).
To create the printed circuit board, the upper and lower films 120, 130 are exposed to light, causing the photosensitive coating on both sides of the panel 100 to be chemically altered. After further processing, select portions of the copper on the panel 100 are etched away in accordance with the circuit patterns on the films 120, 130. This creates a circuit pattern on the upper and lower sides of the panel 100, one pattern corresponding to the lower film 120 and the other pattern corresponding to the upper film 130.
Before the upper and lower films 120, 130 are placed against the panel 100, holes, referred to as circuit holes, are drilled into the panel 100. FIG. 2 illustrates one such circuit hole 210. Ideally, the outer perimeter of the circuit hole 210 is surrounded by a ring of copper, known as the annular ring 220. Copper paths, known as "traces" 230 couple circuit hole 210 to other circuit holes in the panel 100. Because the circuit holes 210 are coupled by copper traces 230, electric signals can travel between the circuit holes 210 and to and from any circuit paths soldered to the traces 230 or the annular rings 220. Circuit paths are often coupled together via the annular rings 220 or the traces 230. Additionally, because many printed circuit boards have several layers of circuitry, wires are often soldered to the annular ring 220 to couple the different layers of circuitry.
The annular rings 220 and the traces 230 are part of the circuit pattern created when the films 120, 130 are exposed to light. Thus, it is important that the upper and lower films 120, 130 be properly positioned with respect to the circuit holes 210 so that the annular rings 220 and the traces 230 match up with the circuit holes 210. The likelihood of an annular ring 220 completely surrounding a circuit hole 210 is known as the annular ring 220 tolerance. As circuit densities increase, it become more important that the annular ring 220 tolerance increases.
One method for increasing annular ring 220 tolerance is to decrease the size of the circuit holes 210. This increases annular ring 220 tolerance because the smaller a circuit hole 210, the greater the likelihood that the circuit hole 210 will be completely surrounded by an annular ring 220. A disadvantage of this method is that smaller drill bits are used to drill smaller holes, and smaller drill bits tend to bend during the drilling process, resulting in inaccurate positioning of the circuit holes 210. Additionally smaller drill bits cannot drill through many panels 100. The drilling process is slow and expensive, and, thus it is advantageous to drill through as many panels 100 at one time as possible. Therefore, it is desirable to have a system and method that increases annular ring tolerance without reducing circuit hole 210 size.